Runoff rain gauge

ABSTRACT

Disclosed is a runoff rain gauge  100  which includes a collector tube  104 , soil infiltration resistance medium  108 , a runoff resistance flow element  113 , a standard rain gauge  102 , and runoff collection tube  114 . Precipitation enters the collector tube  104  and is divided to flow into the infiltration medium  108  and runoff collection tube  114  via flow element  113 . Total precipitation is read in standard rain gauge  102 , runoff in tube  114 , and soil infiltration in medium  108  is calculated by the difference between total precipitation and the runoff. Also disclosed are a runoff rain gauge  100  comprising: a seal leg  150  downstream from the runoff resistance flow element  113 ; a collector tube  104  mounted directly to the ground so that the bottom of the soil infiltration resistance medium  108  contacts the ground surface; and/or a horizontal capillary tube in the runoff resistance flow element  113  to insure self-sealing via capillary forces.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application U.S. Ser. No.10/709,155, filed Apr. 16, 2004, now U.S. Pat. No. 7,066,021, whichclaims the benefit of provisional application U.S. Ser. No. 60/320,123,filed Apr. 18, 2003.

BACKGROUND OF INVENTION

This invention relates to a device for capturing rainfall in both astandard rain gauge and a collector tube incorporating an outflowcircuit for simulating runoff. The invention provides an estimate of therainfall split between soil infiltration and runoff. In addition tomeasuring the total rainfall, the device estimates the amount of runoff,thereby providing an estimate of the soil infiltration by difference.

Prior art rain gauges measure the total amount of precipitation. Thiscan be helpful to serve as a general indicator of how much total rainhas fallen, as well as how often and how much irrigation must beprovided for crops, for example. However, the prior art rain gauges failto take into account how much water has run off from the soil, and howmuch has soaked in. Of course, the rainfall that runs off serves nobenefit in irrigation for crops or other plant growth. There is a needin the art for measuring the amount of rainfall that has run off fromthe soil and/or the amount that has soaked in.

There are a variety of conventional rain gauges available on the market,including such designs as simple clear collecting tubes with rulermarkings, ornate collecting tubes for decorating gardens, non-lineartubes with expanded low-end scales to improve readability and electronicgauges with accuracies to a hundredth of an inch. These devices are alldesigned to make the same measurement, specifically the amount of rainwhich falls in the vicinity of the rain gauge.

Generally, when rainfall reaches the ground, it splits into differentflow paths. A small amount may be retained on plant surfaces or pond insmall depressions along the ground surface. However, the bulk of therainfall either (1) infiltrates the soil surface or (2) flows overlandas runoff. Soil infiltration is of primary interest for agriculture andhomeowners with lawns and gardens (who provide supplemental irrigationfor plants), while the run off measurements are of interest tohydrologists monitoring lake levels, storm drainage and flood plains,for example. Both groups typically estimate the quantity of interestfrom the total rainfall using historically developed correlations orsimple experience.

Infiltration rates are a function of several variables including soiltype, compaction, water content, land slope and plant density, as wellas the water depth above the soil. Infiltration rates are highest whenthe soil is very dry, but as precipitation continues and the soilmoisture content increases, the infiltration rate declines andapproaches a constant rate termed the soil percolation rate. Wheneverthe precipitation rate exceeds the infiltration rate, excess wateraccumulates on the surface and a portion of the excess is lost asrunoff. The split to runoff therefore increases with both rainfallquantity and intensity. It is also apparent that the relative fractionsof infiltration and runoff experienced with a rain event are not onlyexpected to be site specific, but will also vary for identical rainevents (quantity and intensity) based on the effects of recent weatheron soil moisture content.

A desirable feature of a device measuring runoff would be designflexibility to fit various formats, including a remotely monitored,self-draining electronic design. The most popular design for electronicrain gauges utilizes a tipping bucket sensor. The sensor is similar to aseesaw with buckets on each end for collecting water. The sensorcollects water in the elevated bucket, which becomes heavy upon fillingand falls, triggering a signal and dumping the contents. The other sidethen becomes elevated and repeats the cycle. The tipping bucket sensoris simple, robust and accurate, and is therefore attractive for use inthe electronic design for a runoff rain gauge. The device would alsodesirably include good access for reading the gauge and other monitoringand maintenance for all of the various formats.

When a significant percentage of soil infiltration comes fromirrigation, a device measuring runoff would preferably be located insidethe irrigation field to capture the effects of irrigation on soilmoisture. This will usually require mounting the unit close to grade. Arunoff rain gauge that provides a grade level mounting would alsoenhance accuracy by providing an extended core sample of maximum depth.Although an in situ mounting provides these features, lack of elevationfor the core sample could make use of an in situ mounting problematicfor a self-draining unit, since a sump pump would be required to returnthe runoff to grade. An alternate low-level mounting system providingboth elevation for runoff drainage to grade and the benefits of anextended core sample is therefore preferred. Ultimately, further designsimplification is also desirable.

SUMMARY OF INVENTION

The present invention measures both the quantity and quality of rainfallby approximating the amount of runoff, thereby providing an estimate forthe rainfall infiltration into the soil as the difference between themeasured rainfall and runoff.

In one embodiment, the invention provides a runoff rain gauge formeasuring precipitation, soil runoff, and soil infiltration bydifference. The gauge has a collector tube with an opening for receivingprecipitation, an infiltration circuit providing a reference soilinfiltration resistance in communication with the collector tube, arunoff circuit in communication with the collector tube providing runoffcharacteristics of a surface of the reference soil, and a runoffmeasuring device, for example, a runoff collection tube to receiverunoff from the runoff circuit, and a measurement system for readingrunoff to the runoff collection tube. The infiltration circuit caninclude a flow resistance medium. The medium can include a sample fromthe reference soil. The runoff circuit can include an air backflow sealbetween the collector tube and the runoff collection tube. The gauge canalso include a balance line located between the runoff collection tubeand a ground surface to maintain a backpressure head to the runoffcircuit matching a water depth above the ground surface. The balanceline can also include a ground connector attached to the balance linepreferably having one or more apertures at a distal end, wherein thedistal end of the ground connector is in contact with the groundsurface. The gauge can include an insulating shroud located about thecollector tube to shield against insolation and retard moistureevaporation from a soil reference sample. The collector tube can includea removable bottom closure with a preferably integral drain hole. Thebottom closure can include a substantially vertical drip tube. Thecollector tube can include an open bottom cylinder to receive a sampleof the reference soil upon direct insertion of the gauge in thereference soil at ground surface. The collector tube further can includea high level recorder. The high level recorder can include a rod coatedwith a water resistant material painted with a water soluble dye, and acap positioned within the collector tube above the infiltration circuitand having a surface area smaller than a cross-sectional area of thecollector tube. The runoff collection tube can include a drain valve.The gauge can include a standard rain gauge for measuring totalprecipitation.

The runoff rain gauge can also include a frame connected to thecollector tube, a standard rain gauge and the runoff collection tube,wherein the frame is attached to a support to maintain the rain gaugeand collector tube in a vertical orientation with opening above groundlevel to receive precipitation. The runoff gauge can also include anindependently adjustable runoff resistance. The runoff circuit caninclude an upper horizontal tube and a lower horizontal tube, thehorizontal tubes connected by an upstream vertical tube and a downstreamvertical tube. The upper horizontal tube connects an outlet of thecollector tube and an inlet of the runoff collection tube and includes anon-permeable plug therein, the lower horizontal tube includes a filtermedium, and a screw is provided in the downstream vertical tube toadjust flow resistance.

The invention also provides a method for measuring rain runoff. Themethod includes: (a) collecting precipitation in a collector tube; (b)passing a first portion of the collected precipitation to aninfiltration circuit providing infiltration resistance characteristicsof a reference soil; (c) passing a second portion of the collectedprecipitation to a runoff circuit providing runoff characteristics of asurface of the reference soil; (d) collecting the second portion in arunoff collection tube; and (e) measuring the precipitation collected inthe runoff collection tube. The method can further include: (f)developing a pressure imbalance wherein a head of water in the collectortube is greater than the backpressure head in the runoff circuitprovided by a ground level water depth; and (g) passing water from thecollector tube through the runoff circuit to restore pressure balance.

Also provided is a method for measuring rain infiltration in soil,including: (a) measuring total precipitation; (b) measuring rain runoffas described above; and (c) determining infiltration by the differenceof the total precipitation and the rain runoff.

Also provided is a method for calibrating a runoff rain gauge withadjustable runoff resistance. The method includes: measuring an averagemaximum water depth for the reference soil and a maximum water depth forthe collector tube during a rain event; and adjusting the runoffresistance in proportion to any difference between the measured averagemaximum water depth for the reference soil and the maximum water depthfor the collector tube.

The invention also provides a method for measuring maximum water depthswith a high level recorder. The method includes: (a) painting a rod witha water-soluble dye; (b) positioning the rod vertically inside aperforated tube; (c) anchoring the perforated tube on a reference soil;(d) allowing rainwater from a rain event to enter the perforated tubeand dissolve the dye on the rod as the water level increases; (e)allowing the perforated tube to drain once the rain event subsides; (f)measuring the maximum depth of the water based upon the dye remaining onthe rod; and (g) measuring the maximum depth of water on a soilinfiltration surface inside a collector tube of a rain runoff gauge,wherein the gauge includes a painted rod positioned inside the collectortube.

Also provided is a method for cultivating plants growing in soil. Themethod comprises: (a) positioning a rain runoff gauge adjacent a soillocation, wherein the gauge includes a collector tube, an infiltrationcircuit and a runoff circuit, the infiltration circuit providing aninfiltration resistance having characteristics of the soil and therunoff circuit providing runoff characteristics of a surface of thesoil; (b) collecting ambient precipitation in the collector tube; (c)passing a first portion of the collected precipitation through theinfiltration circuit; (d) passing a second portion of the collectedprecipitation through the runoff circuit and collecting theprecipitation in a runoff collection tube; (e) measuring the secondportion of the precipitation passing through the runoff circuit; (f)measuring total precipitation; and (g) irrigating the soil as a functionof the measured runoff and measured precipitation.

In one embodiment, the runoff rain gauge can incorporate a seal leg inthe outflow circuit from the collector tube. The seal leg allowshydraulic decoupling of the gauge measuring runoff from the pressurizedsection of the outflow circuit providing resistance to runoff. An inletsection to the seal leg can be pressurized to provide resistance againstrunoff from the collector tube while sealing off the downstream circuitfrom pressurization.

The seal leg can be a U-tube or use more or less concentric outer andinner upright tubes, with side connections for directing runoff into andout of the seal leg. A side inlet of the seal leg can connect to therunoff line from the collector tube. A side outlet of the seal leg canattach to a vented drip chamber or remain detached for direct drip intothe runoff collection tube. The side outlet can also incorporate a drippoint wherein outflow water drips against atmospheric pressure.

The seal leg can attach to a balance line located between the seal legand a ground surface. The balance line can include a ground connectorattached to the balance line comprised of a base plate with anchor rodin contact with the ground surface and a seal cap with continuousstraight bottom edge. The seal cap can extend substantially beyond thefootprint of the base plate to form a thin, substantially continuous gapbetween the seal cap edge and ground surface.

Runoff from the collector tube in one embodiment of the concentric tubeseal leg arrangement drips against pressure into the inlet section ofthe seal leg, flows through a down-flow area comprising the annularspace between the inner and outer tubes to a bottom of the seal leg,into the inner tube and upwards to a horizontal turn into the sideoutlet connection. Runoff then exits the seal leg by dripping againstatmospheric pressure.

The function of the seal leg can alternatively or additionally beprovided by other similar configurations such as a U-tube seal leg, forexample. The hydraulic performance of the U-tube seal leg is essentiallythe same as that of the concentric tube seal leg, with the inlet side ofthe U-tube functioning as the inlet section and down-flow area, and theoutlet side of the U-tube functioning as the inner tube of theconcentric tube leg.

Another embodiment includes an alternate mounting system for the runoffrain gauge wherein a collector tube containing a previously extractedcore sample is directly coupled to the ground such that the bottom coresurface and top ground surface are in direct contact. This configurationallows the runoff measurement system to be positioned above grade,thereby allowing runoff drainage to grade, while also effectivelyproviding the accuracy of an extended core sample. The ground-coupleddesign is advantageous whenever a low level mounting is required, and ispreferred for a self-draining unit such as an electronic gauge, forexample with a tipping bucket sensor.

Another embodiment provides a self-sealing capillary action tube as theflow element in the runoff line, i.e. a sufficiently small diameter tubeto provide for self-sealing via capillary action, thereby facilitatingavoidance of inlet and outlet vertical barriers in the runoff line forsealing against air backflow.

The flow element can comprise an inner horizontal tube mountedconcentrically in the runoff line, with annular end caps to support theinner tube from the runoff line and seal the annular space between theinner tube and the runoff line, thereby forcing flow through the innertube.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a simplified schematic drawing of the principal components ofa runoff rain gauge in accordance with one embodiment of the presentinvention.

FIG. 1A is a detailed schematic drawing of the runoff line of the gaugeof FIG. 1.

FIG. 2 is a schematic drawing of an alternate embodiment of the gauge ofFIG. 1.

FIG. 2A is a detailed cross-sectional drawing of the seal cap of thegauge of FIG. 2.

FIG. 3 is a schematic drawing of an in situ runoff rain gauge accordingto another embodiment of the invention.

FIG. 4 is a simplified schematic drawing of a runoff rain gauge with anindependently adjustable runoff resistance according to anotherembodiment of the invention.

FIG. 5 is schematic drawing of a high-level recorder for taking groundlevel water measurements according to another embodiment of theinvention.

FIG. 6 is a schematic drawing of a high-level recorder for takingmeasurements in the collector tube according to another embodiment ofthe invention.

FIG. 7 is a schematic drawing of secondary features of the runoff raingauges of FIG. 1 and FIG. 4.

FIG. 8 is a schematic drawing of a runoff resistance circuit accordingto another embodiment of the invention.

FIG. 9 is a simplified schematic drawing of the principal components ofa runoff rain gauge incorporating a seal leg in accordance with oneembodiment of the present invention.

FIG. 10 is a schematic drawing of the principal components of aconcentric tube seal leg that can be utilized in the gauge of FIG. 9.

FIG. 11 is a detailed cross-sectional drawing of the seal cap used withthe seal leg of FIG. 9.

FIG. 12 is a schematic drawing of a U-tube seal leg that can be used asan alternate to a concentric tube seal leg in the gauge of FIG. 9.

FIG. 13 is a schematic drawing of an electronic design for a runoff raingauge incorporating a ground-coupled core sample and vented atmosphericdrip chamber in accordance with another embodiment of the presentinvention.

FIG. 14 is a schematic drawing of one embodiment of the runoff circuitin the runoff line with a capillary tube air seal in accordance with thepresent invention.

DETAILED DESCRIPTION

The present invention provides a precipitation measuring device capableof simultaneously measuring rainfall and soil runoff during a rainevent. Soil infiltration is thereby determined by difference.

FIG. 1 illustrates one principal embodiment of the rainfall measuringinstrument 100. The device consists of standard rain gauge 102,preferably including graduated markings for determination of rainfallamounts. The instrument 100 also includes a collector tube 104, whichincludes a flow resistance device 106. The flow resistance device 106simulates soil infiltration resistance and is preferably comprised of aflow resistance medium 108. Preferably, the flow resistance medium 108is comprised of a soil sample taken from the site being modeled, ofsufficient depth to approximate the natural percolation rate of thesoil. This approach allows the resistance to exhibit natural sensitivityto such factors as recent rains or extended dry periods. The soil plugis typically extracted from the ground by driving the lower barrel ofthe collector tube the required depth into the soil, and then twistingthe barrel to break the plug loose from the ground. Very sandy soilsthat flow easily can simply be scooped into the barrel to provide therequired sample. The bottom closure 110 includes a drain 109, ensuringevaporation from the medium 108 occurs principally from the top surface,while the drain allows the medium to be free draining after saturation.

The device also includes a second (parallel) circuit 112, which exitsthe collector tube 104 directly above the flow resistance medium 108.The runoff line 112 includes flow element 113 which can provideresistance to runoff by maintaining a static pressure head opposing thewater column height in the collector tube 104. As shown in FIG. 1A, flowelement 113 can be comprised of two vertical barriers 115 and 116adjoined in a horizontal tube. The first vertical barrier 115 extendsdown from the top of runoff line 112 to a passage at the bottom. Asecond vertical barrier 116 extends upward from the bottom of runoffline 112 to a passage at a height equal or greater than the bottom ofthe first barrier 115. In use, rain enters collector tube 104, a portionof the rainwater infiltrating the soil medium 108, while the remainingportion enters the runoff line 112. The runoff line 112 may include anylon wick (not shown) or other hydrophilic material to minimizeresistance to flow at low levels. Water entering runoff line 112initially encounters the first vertical barrier 115, which it then flowsunderneath. To flow over the second vertical barrier 116, wateraccumulates to a height above the gap at first barrier 115, sealing offthe air space downstream first vertical barrier 115 and preventingbackflow of air into collector tube 104. As the water accumulates in thearea between the collector tube 104 and the first vertical barrier 115,pressure increases until the water overflows second vertical barrier 116into the runoff collection tube 114. The (horizontal) water level 117between vertical barriers 115 and 116 must rise against the backpressurehead resisting runoff to overflow vertical barrier 116.

Rain runoff is collected in runoff collection tube 114, which desirablyincludes graduated markings for measurement of total runoff during aprecipitation event. The cross-sectional area and scale of collectiontube 114 desirably correlate with the cross-sectional area and scale ofcollector tube 104. Runoff collection tube 114 includes a valve,stopcock, or similar device 116 located at the bottom of the runoffcollection tube 114 for alternately retaining and releasing accumulatedrunoff. Runoff collection tube 114 also includes balance line 118, whichconnects the air space of runoff collection tube 114 to the groundsurface. The balance line 118 serves to maintain a sealed system, whileat the same time communicating the water head existing at the groundsurface.

As runoff flows into runoff collection tube 114, air displaced from thetube 114 exits through the balance line 118, located at or near the topof runoff collection tube 114. As shown in FIG. 2, the balance line 118can be connected to a seal cap 122, secured to the ground surface viarod 124 and base 124(a). The air displaced out of the runoff gaugebubbles out the submerged bottom end of the seal cap 122, throughapertures 123 located at the base of seal cap 122. The seal cap is shownin greater detail in FIG. 2A. The head of water 125 above the apertures123 backpressures the air in the balance line 118, runoff collectiontube 114 and the air space in the runoff line 112. In effect, theresistance to runoff for the modeled terrain (as reflected by the depthof the water at ground level) is automatically transferred from gradelevel to the runoff rain gauge, thereby circumventing the need forcalibration as is required for a stand-alone runoff resistance. Theoutlet of the balance line circuit, as represented by the apertures 123located within seal cap 122, is carefully anchored at grade level withthe rod 124 or other means for securing the seal cap 122 with the base124 a abutting the ground surface below the apertures 123, andaccurately transfers the developing ground-level water depth to the unit100.

In practice, the process wherein runoff is induced to restore hydraulicbalance occurs in very small steps on a relatively continuous basis. Theprocess may occur during all phases of a rain event, including while theground-level water depth is building, while it is holding steady, andwhile it is declining. As water depth declines, imbalances develop asthe ground-level seal declines, rather than as the collector tube 104head builds, causing water stored in the collector tube 104 to run off.Priming the circuit 112 and balance line 118 seals induces a positivepressure in the runoff collection tube 114, requiring proper joint sealsfor an airtight design.

As illustrated in FIG. 2, an alternate embodiment includes a mountingframe consisting of two parallel horizontal bars 126(a), 126(b) and twoparallel vertical bars 127(a), 127(b) (127(b) not shown) which securethe standard rain gauge 102, the collector tube 104, and the runoffcollection tube 114. The frame is mounted to a vertical rod 120,allowing the device to be oriented in any direction desired by theoperator. Alternatively, a shorter vertical rod can be used withappropriate clamps for mounting the device to a stationary object, suchas for example a fence.

FIG. 3 illustrates an alternate embodiment of the rain and runoffmeasurement device wherein in situ measurements of the runoff arerecorded. This design does not require extraction of a core soil sampleto simulate the soil infiltration resistance. Instead, the in situ gaugeis mounted at grade level by inserting the collector tube 104 directlyinto the ground. The inlet of circuit 112 is preferably positioned atgrade level. The runoff circuit 112 preferably creates an air backflowseal loop 132, shown in FIG. 3 as a U-shaped tube. Runoff collectiontube 114 is mounted below grade, preferably in a holder 136. Runoffcollection tube 114 is connected to runoff line 112 with a simplecoupling, allowing easy removal of tube 114 to read the collected runoffand/or empty the contents. The device also includes a balance line 118to read ground water depth. Performance of the balance line 118 has beenpreviously described. The in situ device has the advantage of mostaccurately maintaining the original conditions of the soil infiltrationsample.

FIG. 4 illustrates another embodiment of the design for arainfall-measuring instrument, which provides an estimate of therainfall split between soil infiltration and runoff. In addition tomeasuring the total rainfall, the device estimates the amount of runoff,thereby providing an estimate of the infiltration by difference.Rainfall is captured in both a standard rain gauge 102 and a collectortube 104. The lower section of the collector tube contains a flowresistance device 106 to simulate the soil infiltration resistance. Thepreferred device for providing this resistance is to use a flowresistance medium 108 comprised of a plug of the upper soil layer ofinterest. A removable bottom closure 110 is included for collector tube104 to ensure evaporation from the medium occurs principally from thetop medium surface. The bottom closure 110 includes an integral drainhole 109 to allow for free drainage of the medium 108.

The device also includes a second (parallel) circuit 112, exiting thecollector tube 104 immediately above the flow resistance medium 108,which contains a flow element 113 serving as an independently adjustableresistance to simulate the resistance to runoff. The flow element 113 isdesirably adjustable to simulate the local effects of slope, flow pathlength and roughness (vegetation density) on water depth. Flow throughthe runoff circuit 112 is collected in a runoff collection tube 114whose cross-sectional area and scale desirably correlate with the areaof collector tube 104. A cover or hood 140 is attached to runoffcollection tube 114 to prevent rainfall from entering tube 114.Electronic rain gauges can be used for remote monitoring and interfacingto automated control systems.

To provide useful information, the two parallel flow resistances in theunit, the soil infiltration circuit and the runoff resistance circuit,must accurately represent the corresponding resistances to soilinfiltration and runoff for the area of interest. The accuracy of thesoil infiltration resistance can be insured by incorporating a sample ofthe soil of interest into the unit. The accuracy of the runoffresistance is more difficult to provide. In the embodiment of FIG. 4, anadjustable flow element 113 is calibrated to provide a resistance, whichpreferably results in the same level of water developing on top of thesoil sample during a rain event as the average level developed on theground area of interest. This embodiment benefits from the simultaneousmeasurements of water depths at multiple locations across a desired areaas well as in the unit itself, and subsequent calibration of the runoffmeasurement device.

If the area being modeled by a single unit is not too large, the maximumwater depths experienced at all locations will occur near the same timein a rain event, typically near the end of the most intense period ofrain. Measuring the maximum water levels which develop at variouslocations across the area being sampled, as well as inside the collectortube 104, provides simultaneous measurement of the water levels in boththe modeled area and the unit.

Several measurements are desirably taken to provide a good approximationof the average depth for the area of interest. FIGS. 5 and 6 illustratedesigns for simple yet effective high-level recorders for measuring themaximum water depth developed at a specific location and during aspecific precipitation event. The design 180 for recording groundlevels, shown in FIG. 5, has a hollow tube 181, closed at both ends,with holes located at the bottom 184 and top 183 of the tube 181. Abottom hole or aperture 184 allows water to enter and leave the tube181, while the top hole 183 serves as a vent, allowing air within thetube 181 to be displaced. The top closure 185 is removable, and has adipstick rod 182 attached to it of about the same length as the tubeheight. The rod 182 is covered with a textured water-repellant materialand painted with a water-soluble dye 188 of a color that contrasts wellwith the water-repellant material covering the rod 182. A second rod187, or similar device, extends from the bottom closure 186, and anchorsthe unit to the top of the soil.

The high level recorder 180 is positioned by holding the gaugevertically and inserting the bottom rod 187 into the ground until thebottom tube closure 186 stops flush with the ground. The bottom rod 187holds the gauge 180 securely in place. The design buffers the interiorof the gauge 180 from waves and splashing external to the tube,providing a stable internal level. As the water level rises in therecorder 180, the water-soluble dye 188 is washed off the dipstick 182,leaving a clear interface on the rod 182 designating the highest levelreached during the rain event. As the rain subsides and the externallevel drops, the water and washed-off dye flow out the bottom hole 184.A defined color interface is produced on the rod since (1) the internallevel is kept stable and (2) the coating on the rod 182 is waterrepellant and therefore does not smear the interface by wicking thewater upwards.

FIG. 6 illustrates another embodiment for recording high levels in theunit. In this embodiment, a dipstick rod 182 is mounted to the center ofthe top debris screen 148. The embodiment includes a hat or cover 160immediately below the screen 148 to divert rainwater toward the wall ofthe collector tube 104, preventing water from running down the dipstickrod 182. The screen 148 and hat 160 also prevent direct impact of rainon the water surface, thereby preventing splashing and insuring a stablewater level.

The high level recorders do not require maintenance in their use duringa rain event. Prior to the rain event, the dipstick rod is painted withthe water-soluble dye. After the event, the high level recorders arecollected, the interface levels are measured and averaged, and theresult is compared to the maximum level measured in the unit's collectortube 104.

FIG. 7 illustrates some secondary features for the rainwater measuringdevice, including an overflow tube 145 to limit the maximum water depthwhere desired (e.g. limit water depth to the grass cut level on lawns),and a pivoting mount 142 with bottom anchor to provide for easy drainingof the gauges. A shroud 128, having internal insulation 129, shields thesides of the collector tube 104 from direct sunlight, preventing excessevaporation. Evaporation can be further moderated with an internal pan144 and downtube 146 to reduce the available area for diffusion andconvective cell formation. Screens 147, 148 and 149 are used to bufferraindrop impact, support the soil sample, and keep debris out of therunoff stream.

The collector tube bottom closure 110 ensures evaporation occursprincipally from the top surface of the resistance medium 108. A drainhole in the bottom closure 110 allows the medium 108 to drain freely.Drainage occurs whenever the wetting front moving through the medium 108reaches the bottom of the medium bed, corresponding to full saturationof the bed. For media exhibiting hygroscopic properties, as is typicalwith most soils, full saturation can result in a sudden drop ininfiltration rate due to the loss of the capillary suction force in themedium 108, typically referred to as the wetting front suction head. Thedrop in infiltration rate can be suppressed by including a substantiallyvertical drip tube 111, in lieu of a simple drain hole, for the bottomclosure. When drainage to the bottom closure commences, the drip tube111 primes up and induces a vacuum to the bottom of the resistancemedium 108, significantly compensating for the loss of the wetting frontsuction head while still providing for free drainage of the medium bed.

The water depth in the collector tube affects both the driving force forinfiltration and the total time during which a water layer is availableto supply infiltration. The depth controlled by the runoff circuit is afunction of the localized runoff alone, as compared to the land area ofinterest where depth at any point is a function of hydraulic gradientsresulting from the cumulative runoff for the surrounding area. However,the two depths are closely linked as direct responses to excess rainfallexceeding the infiltration rate, and therefore can be proportionallycorrelated.

Water has a relatively high surface tension, which creates difficultiesin designing a flow circuit to operate with a very low head. The watertends to be reluctant to flow onto dry (non-hygroscopic) surfaces andrelease from wet surfaces. Significant head, or differential waterpressure, may be required to initiate flow through a dry fine meshfilter. Flow components with small cross-sectional areas frequentlydevelop a form of vapor lock at the inlet end during dryout, requiringsignificant head to drive the resulting bubble through the circuit whenwater flow is reestablished.

An alternate embodiment of the flow resistance device developed for anindependently adjustable runoff resistance component 113 addresses theseconcerns, insuring that priming and flow is established at very lowwater depths and in various stages of unit dryout. The circuit design200 is illustrated in FIG. 8, and employs two horizontal and twovertical tubes. The top horizontal tube 203 is comprised of inletsection 202 and outlet section 204 and is partitioned with a simple plug206. Flow enters the top horizontal tube 203 through inlet 202 and flowsdown the vertical downtube 208 to the bottom horizontal tube 210. Thestream flows into the bottom horizontal tube 210 and through a filterelement 212, which protects the downstream flow element 216 againstplugging. Flow then enters the bottom of the vertical uptube 214, whichincludes flow control element 216, which comprises the bulk of theflowing resistance for the circuit 200. Water flows upwards through theelement 216 and returns to the top horizontal tube 203 where itdisengages from the flow resistance element 216 and flows towards theoutlet end 204 into a downstream runoff collection tube 114 (see FIG. 7)for collecting the runoff.

The design of the circuit 200 allows the unit to develop significantavailable head (difference in liquid levels in the two vertical tubes)for initiating flow through both the filter 212 and flow element 216.Head develops once spillover into the downtube 208 begins, and increasesas the level in the downtube 208 rises. Once full flow is establishedand the waterfront reaches the outlet 218 of the flow element 216, thenet head through the unit decreases to essentially the available head atthe inlet 209 of the downtube 208. The vertical upflow design of theflow element 216 insures the element will dry out from the outlet 218towards the inlet 215, further guarding against vapor lock if the unitprimes up before complete dry out.

The flow element 216 is preferably designed to exhibit laminar flowwhereby the relation between flow and head is linear. This more closelyapproximates the relation between runoff rate and depth than would beseen in turbulent flow, where head varies with the square of the flow.Other approaches such as thin tubes, packed tubes or narrow spilloverweirs could also be used to provide a suitable response to runoff. Thintubes and packed tubes would again produce a laminar response, while anarrow spill-over weir would produce a milder response of head to flowwhere head varies by a factor of an exponent of 2/3 on the flow. Inpacked tubes, the resistance medium must be carefully selected toproperly simulate the flow resistance exhibited by the terrain.Spillover weirs may better approximate the relation between flow andhead in certain cases such as for relatively smooth surfaces (i.e. thinvegetation) as well as level surfaces where the principal driving forcefor runoff is water depth rather than slope. However, the relativelylarge capacity of even a very narrow weir slot, rectangular ortriangular in shape, would require the use of a much larger collectortube and soil sample to load up the slot sufficiently to generateaccurate water depths. The use of a laminar element for these cases isstill considered acceptable since infiltration rate is not a strongfunction of water depth, so moderate deviations in unit water depth willnot significantly affect the simulated runoff.

The design for the flow element 216 illustrated in FIG. 8 incorporates ascrew-in-tube-type design, such that the screw threads tightly fit, orscore, the inside of the tube wall, sealing against bypass. Flowtherefore spirals upwards along the groove between the threads. Theresistance is adjusted by turning the screw to drive it into or out ofthe surrounding tube, thereby increasing or decreasing the flow pathlength through the element. A similar type of effect can be achievedwith a bolt in a threaded tube or coupling, wherein the ends of the boltthreads and/or tube threads are partially ground off to create the flowpath. This provides a good seal between threads while avoiding dustformation, which can occur when the resistance of the screw in tubedesign is adjusted. Additionally, removable transparent end caps 220attached to the ends of the bottom horizontal tube 210 facilitateinspection and cleaning of the filter element 212.

The design illustrated in FIG. 8 makes calibration of the unit simple,because the resistance is linearly proportional to the active screwlength. After a rain event, level recorders 180 are collected, theinterface levels are measured and averaged, and the result compared tothe maximum level measured in the collector tube 104. The resistance isthen adjusted as required to raise or lower the level that would developif the same rain event were repeated.

FIG. 9 illustrates the principal components of a runoff rain gauge 100in accordance with one embodiment of the invention. The designincorporates a seal leg 150 in the runoff outflow circuit from thecollector tube 104. The seal leg 150 hydraulically decouples therunoff-measuring gauge 114 from the pressurized section of the outflowcircuit providing resistance to runoff. The seal leg 150 can bepressurized to provide resistance against runoff from the collector tube104. A balance line 118 connects the top of the seal leg 150 to a sealcap 122 located at ground level. The seal leg 150 receives flow from therunoff line 112 and flow element 113 (discussed in more detail below inconnection with FIG. 14) and discharges flow to the runoff collectiontube 114. A cover or hood 140 can be located above runoff collectiontube 114 to exclude rainfall.

FIG. 10 illustrates the principal components of a concentric tube sealleg 150A that can be used for the seal leg 150 service. The concentrictube seal leg 150A can include an outer upright tube 151 and inner tube152 having an upright portion defining an annulus between the tubes 151,152, with lateral connections for directing runoff into and out of theconcentric tube seal leg 150A. The side inlet 155 can hydraulicallyconnect the concentric tube seal leg 150A to the runoff line 112 fromthe collector tube 104. The side outlet 156 can include a lateralportion of the inner tube 152 extended transversely through a seal toterminate at a detached end 154 for direct drip into the runoffcollection tube 114. The side outlet 156 thus provides a drip point atend 154 wherein outflow water from the inner tube 152 drips againstatmospheric pressure.

The inner tube 152 can have an open bottom end adjacent the bottom ofthe outer tube 151, and can incorporate an elbow, e.g. a 90° elbow, toallow the upper part of the tube 152 to extend transversely through theside outlet 156. The top closure 157 of the outer tube 151 can attach tothe balance line 118 hydraulically located between the concentric tubeseal leg 150A and a ground surface. The outer upright tube 151 caninclude a preferably removable bottom cap 153 to facilitate cleanout ofthe concentric tube seal leg 150A.

The balance line 118 can attach to a ground connector as detailed inFIG. 11. The ground connector in this example is comprised of a baseplate 124 a with anchor rod 124 in contact with the ground surface and aseal cap 122 with continuous straight edge extending substantiallybeyond the footprint of the base plate 124 a. The thin, substantiallycontinuous gap formed between the bottom edge of the seal cap 122 andground surface provides for hydraulic coupling of the balance line toany liquid head over the gap.

Designs for the runoff rain gauge 100 incorporating a seal leg 150 donot store runoff in the pressurized section of the outflow circuit, andtherefore do not induce bubbling from the ground level seal cap 122. Thecap 122 therefore uses a continuous or straight peripheral bottom edge,compared to the aperture notches on the cap bottom edge of FIGS. 2 and2A, with minimal clearance to the ground to hydraulically couple withany liquid head present on the ground at the cap 122. Runoff iscollected and/or measured downstream of the atmospheric drip point 154exit the seal leg 150.

A rising ground water level 125 seals the ground level seal cap 122 andpressurizes the air pocket inside the cap 122. Air displaced up thebalance line 118 pressurizes the inlet section of the concentric tubeseal leg 150A, pushing the annular water level 158 below the elevationof the atmospheric drip point 154. Water from the collector tube 104flows through the runoff line 112, drips against pressure in the inletsection of the concentric tube seal leg 150A, flows downward through theannular space between the outer tube 151 and inner tube 152, flows upthrough the inner tube 152 and exits via dripping against atmosphericpressure.

An important consideration is the ratio between the horizontal areainside the ground level seal cap 122 and the annular area comprising thecross-sectional down-flow area in the concentric tube seal leg 150A. Thelarger the ratio, the smaller the error induced by having a water levelunderneath the seal cap 122 higher than the edge of the cap 122. Forexample, using a design with an area ratio of 3:1 means that when theground water level is 1″ high, the water level underneath the seal cap122 will come up about 0.25″ above the cap 122 edge, resulting in about0.75″ of water head pressurization in the concentric tube seal leg 150A,thereby supporting an equivalent water height in the collector tube 104.The error in the transmitted ground water level is therefore about 25%.In one preferred embodiment, the design for the concentric tube seal leg150A and cap 122 provides an area ratio of at least about 4:1, at leastabout 10:1, or at least about 100:1, corresponding to respective errorsof only 20%, 9% or 1% in the transmitted ground water level.

The function of the seal leg 150 can also be provided by other similarconfigurations such as the U-tube seal leg 170 illustrated in FIG. 12.The hydraulic performance of the U-tube seal leg 170 is essentially thesame as that of the concentric tube seal leg 150A, with the inlet side171 of the U-tube 170 functioning as the outer tube 151 and annularspace of the leg 150A, and the outlet side 172 of the U-tube 170functioning as the inner tube 152 of the leg 150A. Previous commentsregarding the performance of the concentric tube seal leg 150A aresimilarly applicable to the U-tube seal leg 170 using thiscorrespondence. While the concentric tube seal leg 150A is generallypreferred over the U-tube seal leg 170 since, for a given tube size forboth the U-tube 170 and seal leg 150A outer tube 151, the annular areaof the leg 150A is less than the cross-sectional area of the U-tube 170and ground water level is therefore more accurately conveyed in the sealleg 150A than the U-tube 170, the U-tube 170 can be advantageous forsimplicity of construction where accuracy is less important and/or wherethe U-tube 170 has a smaller tube diameter.

FIG. 13 illustrates another embodiment of the invention including a lowlevel mounting that is beneficial for use with an electronic design forthe rain and runoff measurement device. An alternate mounting system isshown wherein a core soil sample 108 is placed into the collector tube104, and the collector tube 104 containing the core soil sample 108 isdirectly coupled to the ground such that the bottom surface of the core108 and top ground surface are in direct contact. This configurationelevates the top surface of the core 108 above grade, allowing runoffdrainage to grade, while also effectively providing the accuracy of anextended core sample. The length of the collector tube 104 below therunoff line 112 is sufficient to encompass the targeted height of theextracted core sample 108 plus the required submergence below grade formechanical stability. The design is applicable whenever a low-levelmounting is required, and is preferred for the self-draining electronicformat.

For example, for a gauge mounted with an 8″ core sample 108 directlycoupled to the ground and 4″ submergence of the collector tube 104 formechanical support, a segment length of about 12″ from bottom of thecollector tube 104 to the outlet connection for the runoff line 112 isrequired. The collector tube 104 is first inserted 8″ into the ground toextract a core sample 108. The core is preferably taken from soil nearthe desired mounting site having similar characteristics to the soil atthe mounting site, but at a sufficient distance so as not to impactgauge performance or installation at the mounting site. The tube 104 isthen positioned at the mounting site and inserted 4″ into the ground. Asthe tube 104 is inserted into the soil at the mounting location, theground surface can push the core sample 108 up an additional 4″ untilthe top surface of the core 108 adjoins the outlet connection for therunoff line 112. Water percolating through the core sample 108 continuespercolating downwards into the ground.

The gauge 102 for measuring total precipitation can be a standardelectronic rain gauge using a tipping bucket sensor 119. The runoffmeasurement device 114 can be a standard electronic rain gauge with acovered top to prevent rainfall from entering. The electronic gauge canbe self-draining to minimize maintenance requirements.

The outflow circuit can incorporate a seal leg 150 in accordance withFIG. 9. For the embodiment shown in FIG. 13, the side outlet can beconnected to a vented drip chamber 158. A runoff drain line 159 canconnect the drip chamber 158 with the runoff measurement device 114.

Runoff from the seal leg 150 drips against atmospheric pressure into thedrip chamber 158, and then drains via gravity flow through the runoffdrain line 159 into the runoff measurement device 114. Use of a flexibledrain line 159 eases the spacing tolerance between the collector tube104 and gauge 114, simplifying installation.

FIG. 14 illustrates another embodiment of the invention utilizing a tubeof sufficiently small diameter for the flow element 113 in the runoffline 112 to provide for self-sealing via capillary action on the tubewalls. A sufficiently small tube cannot support an internal vapor-liquidinterface, requiring the tube to operate either dry or flooded. Such acapillary tube therefore does not require the use of vertical barriersat the inlet and outlet to seal against the backflow of pressurized airfrom the seal leg 150 to the collector tube 104.

The flow element 113 shown includes a small diameter tube 191 mountedconcentrically inside the runoff line 112. Annular end caps 193 a, 193 bserve to support the inner tube 191 in the runoff line 112 and seal theannular space between the tubes, thereby forcing flow through the innertube 191.

The invention is described above in reference to specific examples andembodiments. The metes and bounds of the invention are not to be limitedby the foregoing disclosure, which is illustrative only, but should bedetermined in accordance with the full scope and spirit of the appendedclaims.

1. A rain runoff gauge, comprising: a collector tube having an openingfor receiving precipitation; an infiltration circuit providing areference soil infiltration resistance in communication with thecollector tube; a runoff circuit in communication with the collectortube providing runoff characteristics of a surface of the referencesoil; a runoff measuring device to measure runoff from the runoffcircuit.
 2. The runoff gauge of claim 1, wherein the runoff circuitcomprises a seal leg for operation of the runoff measuring device atatmospheric pressure.
 3. The runoff gauge of claim 2, wherein the sealleg comprises: an upright outer tube having a side inlet connection,side outlet connection, top closure and bottom removable cap; an innertube with an inlet end disposed within the outer tube below the sideoutlet connection, and an upper part extending transversely through theoutlet connection of the outer tube.
 4. The runoff gauge of claim 2,further comprising a balance line attached between an inlet to the sealleg and a ground surface to maintain a backpressure head in the runoffcircuit matching a water depth above the ground surface.
 5. The runoffgauge of claim 4 comprising a seal cap at an end of the balance line atthe ground surface.
 6. The runoff gauge of claim 5 wherein the seal capprovides an internal horizontal area at a ratio to an internalhorizontal down-flow area of the seal leg that is at least 4:1.
 7. Therunoff gauge of claim 2, wherein the seal leg comprises: a U-tube, sideinlet connection, side outlet connection, top closure for the tube endabove the inlet, and removable cap for the tube end above the outlet. 8.The runoff gauge of claim 1, wherein the infiltration circuit includes areference soil specimen.
 9. The runoff gauge of claim 8 wherein a bottomof the reference soil specimen is in direct contact with a groundsurface.
 10. The runoff gauge of claim 1, wherein the runoff circuitincludes an air backflow seal between the collector tube and the runoffmeasuring device.
 11. The runoff gauge of claim 10, wherein the airbackflow seal includes a capillary tube.